Invalid subnet masks

[Eric van Gyzen]

On 02/11/2015 05:51, Julian Elischer wrote:
> On 2/11/15 5:55 PM, Matt Churchyard wrote:
>
>>
>> Are there actually valid use cases for these types of network?
> yes.
> I've had networks that were the first and last quarter of a /24, and
> the middle two quarters were separate nets.
>
> Sure, it made my skin crawl, but I was in a pinch to get more machines
> onto that /26.
> all four were served by the same router so only one router needed to
> know..
>
> I have however at times though we could think about making ifconfig at
> give a warning.
> (but not an error).
>
>> I'm learning towards the opinion that they should be rejected unless
>> the user specifically overrides it (with something like an ifconfig
>> flag or sysctl).

These valid use cases are so rare, I would favor making this an error in
ifconfig, but also providing a flag to silence the message and accept
the mask.  The error message could even mention the name of the flag, to
be helpful.  For example:

# ifconfig igb0 netmask 250.250.250.0
ifconfig: netmask should be contiguous and left-justified; specify
"incontiguous" to override
# ifconfig igb0 incontiguous netmask 250.250.250.0
#

If it's just a warning, that warning will get very annoying to people
who are forced to use such a mask.  (They're already forced to use such
a network!)

Eric